One technique for cutting workpieces such as metal panels and highly contoured composite panels used as air frame components involves the use of fluid abrasive cutting systems. These systems employ an injector nozzle which dispenses a liquid, such as water, entraining an abrasive material at extremely high pressures. The operating pressures of such systems normally range from 30,000 to 60,000 psi. In a typical application, the high pressure liquid flowing through the nozzle induces a vacuum in a supply line leading to a source of an abrasive grit such as garnet, silica, alumina or the like. Typical abrasive constituents include 100 mesh abrasives for composite materials such as composite laminates of graphite-epoxy or Kevlar Fiber reinforced resins and 60-80 mesh abrasives for metals such as titanium and aluminum.
Systems for the control of the high pressure nozzle is it moves relative to the workpiece being cut range from the simple to the very sophisticated. A relatively simple arrangement using a manually movable nozzle mounted in a guide to bevel the edge of a workpiece is disclosed in U.S. Pat. No. 4,658,683 to Phillips. In Phillips, the nozzle is secured in a collar which is mounted in a guide having a U-shaped configuration. The nozzle is mounted relative to the guide at a appropriate angle to form a beveled surface along the edge of the workpiece. The U-shaped guide is provided with two arms which straddle the position at which the jet is directed at the workpiece and terminate in down turned foot sections so as to position the cutting jet accurately and ensure that it cuts along a path which is parallel to the front face of the workpiece.
Another system which employs a spreading table and conveyor system by which the workpiece may be moved relative to the nozzle is disclosed in U.S. Pat. No. 3,978,748 to Leslie, et al. In this system, a nozzle is mounted on a suitable carriage which is movable along rails of a transport mechanism above a suitable workpiece support such as a wire net tensioned on rollers. A catcher tube is mounted below the wire net opposite the nozzle. The nozzle support is provided with a sensor in the form of having an extendable probe which can be lowered to detect the workpiece surface. A hydraulic mechanism is used to raise and lower the nozzle to provide the appropriate standoff distance based upon the operation of the sensor probe.
A substantially more sophisticated system for robotically controlled abrasive jet cutting is disclosed in Earle, III, George A., "Automatic Trimming of Composite Panels", SAE Paper No. 861,675, October 1986, Society of Automotive Engineers. As disclosed there, the robotic cutting system involves a cutting head which is moved relative to a workpiece by operation of a six-axis gantry robot system. The workpiece to be cut is placed on a suitable support surface and the location of the workpiece relative to the gantry system is accurately determined by a visual control system which senses targets in the workpiece and makes appropriate changes in the program matrix to accommodate the actual position of the workpiece. The cutting head can be moved under the control of a central controller through a three axis cartesian coordinate system to arrive at the desired location after which movement through pitch and yaw axes can be employed to arrive at the desired orientation of the cutting nozzle relative to the workpiece surface to be cut. Movement along a sixth axis coincident with the cutting axis of the nozzle can also be employed.